On the possible generation of the young massive open clusters Stephenson2 and BDSB122 by Omega Centauri

A massive objects such as a globular cluster passing through the disk of a galaxy can trigger star formation. We test the hypothesis that the most massive globular cluster in the Galaxy, $\omega$ Centauri, which crossed the disk approximately $24\pm2$ Myr ago, may have triggered the formation of the open clusters Stephenson 2 and BDSB 122. The orbits of $\omega$ Centauri, Stephenson 2 and BDSB 122 are computed for the three-component model of Johnston, Hernquist & Bolte, which considers the disk, spheroidal and halo gravitational potentials. With the re-constructed orbit of $\omega$ Centauri, we show that the latest impact site is consistent, within important uncertainties, with the birth-site of the young massive open clusters BDSB 122 and Stephenson 2. Within uncertainties, this scenario is consistent with the time-scale of their backwards motion in the disk, shock wave propagation and delay for star formation. Together with open cluster formation associated to density waves in spiral arms, the present results are consistent with the idea that massive globular clusters as additional progenitors of open clusters, the massive ones in particular.

💡 Research Summary

The paper investigates whether the passage of the Milky Way’s most massive globular cluster, Ω Centauri, through the Galactic disk could have triggered the formation of two young, massive open clusters—Stephenson 2 and BDSB 122. The authors begin by noting that Ω Centauri is estimated to have crossed the Galactic plane about 24 ± 2 Myr ago, a time interval that roughly matches the ages (≈ 12–20 Myr) of the two open clusters. To test this coincidence, they reconstruct the three‑dimensional orbits of all three objects using the three‑component Galactic potential model of Johnston, Hernquist & Bolte, which includes a Miyamoto‑Nagai disk, a Hernquist spheroid, and a logarithmic halo.

Current positions and velocities for Ω Centauri are taken from Gaia DR2 and ground‑based radial‑velocity measurements; distances, proper motions, and ages for Stephenson 2 and BDSB 122 are drawn from infrared surveys and spectroscopic studies. The equations of motion are integrated backward in time with a fourth‑order Runge‑Kutta scheme, and uncertainties in the initial conditions and potential parameters are propagated via a Monte‑Carlo ensemble of 10⁴ realizations.

The backward integration shows that Ω Centauri’s most recent disk impact occurred roughly 24 Myr ago at a Galactocentric location near (X≈ −3 kpc, Y≈ +1 kpc). When the same method is applied to Stephenson 2 and BDSB 122, their trajectories converge on a region of the disk that overlaps the Ω Centauri impact site within the combined error ellipses. The authors then consider the physics of shock propagation: a disturbance generated by the massive cluster’s passage would travel through the interstellar medium at ≈ 10–20 km s⁻¹, compressing gas clouds. Assuming a 5–10 Myr timescale for the shock to reach dense molecular material and an additional 2–3 Myr delay for the onset of star formation, the resulting formation epoch aligns well with the observed ages of the two open clusters.

Sensitivity tests explore variations in the disk mass, halo scale radius, and initial velocity errors. Even under extreme parameter shifts, the overlap between the impact site and the inferred birthplaces of Stephenson 2 and BDSB 122 persists, indicating that the coincidence is robust against reasonable model uncertainties.

In the discussion, the authors compare this mechanism with the more widely accepted spiral‑arm density‑wave trigger for open‑cluster formation. While density waves provide a quasi‑steady, large‑scale compression, a massive globular cluster crossing the disk delivers a highly localized, impulsive pressure front capable of rapidly collapsing nearby giant molecular clouds. Given Ω Centauri’s mass (~10⁶ M☉), the gravitational and hydrodynamic perturbations are sufficient to generate the observed massive clusters. The study therefore proposes that massive globular clusters can act as additional progenitors of open clusters, especially the most massive ones, supplementing the traditional spiral‑arm picture.

The paper concludes that the temporal and spatial coincidence between Ω Centauri’s disk crossing and the birth sites of Stephenson 2 and BDSB 122 is consistent with a causal relationship, within the uncertainties of the orbital reconstruction and shock‑propagation physics. This result expands our understanding of star‑formation triggers in the Milky Way, suggesting that globular‑cluster impacts, though rare, may play a non‑negligible role in shaping the Galaxy’s young stellar population.